Beta-alumina is an alumina compound containing an alkali metal oxide, usually as sodium oxide. The sodium ions become highly mobile at about 300.degree. C. which makes .beta.-alumina useful as a solid electrolyte at moderately elevated temperatures. It can be used, for example, as a sodium membrane in a sodium sulfur battery. Articles of .beta.-alumina have been made by extrusion, isostatic pressing, and slip casting. Coatings of .beta.-alumina can be prepared by a sol-gel process or by firing an aqueous slurry of .beta.-alumina. However, these processes for preparing .beta.-alumina coatings are not entirely satisfactory because the sol-gel process is suitable only for making very thin coatings and the slurry process is very slow and requires the substrate to be heated, which may damage certain substrates. Attempts to form coatings of .beta.-alumina using plasma spray torches have failed because the resulting coatings were non-conductive.